In use, fuel nozzles of the piloted or dual orifice type used in gas turbine engines have exhibited a two-fold fouling problem comprising internal coking and external coking which may adversely affect operation and performance of the nozzle. Internal coking is evidenced by carbonaceous type deposits on internal fuel passages resulting from chemical breakdown of fuel from excess heat and residence time in the nozzle. U.S. Pat. No. 4,362,022 issued Dec. 7, 1982 and U.S. Pat. No. 4,111,369 issued Sept. 5, 1978, disclose dual orifice nozzle constructions attempting to overcome internal coking in the internal secondary fuel passage of the nozzle.
External coking or carboning appears on the hot external surfaces of the nozzle face also in the form of carbonaceous type deposits resulting from distillation and/or liquid phase reactions of fuel on the face surfaces. External coking as a result of distillation involves buildup of residues with boiling points above local surface temperatures whereas liquid phase reactions involve other phenomena.
U.S. Pat. No. 3,498,059 issued Mar. 3, 1970 discloses a fuel nozzle for a gas turbine engine constructed to prevent carboning in operation in the engine combustor.
U.S. Pat. No. 4,139,157 listed above describes clogging of primary fuel flow passages as a result of carbonizing of residual fuel therein when primary fuel flow is discontinued.
Piloted or dual orifice fuel nozzles, i.e., nozzles having a pressure atomized low flow fuel discharge past a primary nozzle orifice for engine start-up and low power operation and having a pressure atomized and/or air-atomized high fuel flow discharge through a secondary nozzle orifice for high power engine operation, are shown in U.S. Pat. No. 2,703,260 issued Mar. 1, 1955; U.S. Pat. No. 2,954,172 issued Sept. 27, 1960; U.S. Pat. No. 3,443,760 issued May 13, 69; U.S. Pat. No. 3,520,480 issued July 14, 1970; U.S. Pat. No. 3,684,186 issued Aug. 15, 1972; U.S. Pat. No. 3,785,570 issued Jan. 15, 1974; U.S. Pat. No. 4,105,163 issued Aug. 8, 1978 and U.S. Pat. No. 4,139,157 issued Feb. 13, 1979. British published application 2,022,811 discloses a dual orifice hybrid fuel nozzle for a gas turbine. When a pressure atomized low or primary fuel discharge is employed with an air-atomized (either air-assisted or air-blast) high or secondary fuel discharge from respective orifices, the fuel nozzle is commonly referred to as a hybrid type.
U.S. Pat. No. 3,684,186 listed above illustrates a central pilot or primary fuel conduit having a heat shield on the discharge end and a downstream air diverter with a closed downstream end on the heat shield. However, the nozzle is said to be designed to effect burning close to the nozzle face and to this end the exposed faces of the primary orifice and heat shield are normal to the longitudinal axis of the nozzle.
The aerodynamics of a spiral-swirling turbulent fluid jet emerging from the orifice of a swirl nozzle are described in technical publication Combustion Aerodynamics, p. 102, 1983. There it is indicated that for strong spiral swirl, an adverse axial pressure gradient is sufficiently large to result in reverse flow along the axis of the spiraling flow field and the setting up of an internal central recirculation zone or core of torroidal shape and relatively low pressure in the spiraling jet emanating from the burner nozzle. The recirculation zone constitutes a hot zone of combustion products and unburned fuel. A swirling flow field with a swirl number S' greater than 0.6 is said to be necessary to develop the recirculation zone while a swirl number less than 0.6 is said not to provide axial pressure gradients sufficiently large to cause internal recirculation unless there is a blockage such as a stabilizer bluff body, fuel pipe or pressure jet oil gun in the nozzle, which is instrumental in establishing a recirculation zone. The presence of the recirculation zone plays an important role in flame stabilization in the burner and flame parameters, such as stability, combustion intensity and residence time distribution in combustors, depend on the strength of the vortex of the recirculation zone.